1. Field of the Invention
The present invention relates to an image forming process which adapts a heat transfer process, and an image forming apparatus used for the imaging forming process.
2. Description of the Related Art
A heat transfer process is one of the processes used for forming an image on a transfer body such as paper, a film, or the like. In the heat transfer process, an image is formed on an image receiving sheet by the following steps: superposing a transfer body (a transfer sheet), in which a transfer layer comprising pigment-based dye which is a transfer material is provided on a surface of a substrate, with an image receptor (image receiving sheet) such as paper, a film, or the like; heating the superposed sheets image-wisely from the back surface side of the substrate of the transfer sheet with a thermal head, a laser head, or the like; and transferring the dye onto the image receiving sheet. In another version of the heat transfer process, a dye which is disposed on a transfer body is sublimated by heating and transferred to an image receiving sheet.
There is a disadvantage in the heat transfer process in that, when using a thermal head for image-wise heating, the temperature distribution tends to become uneven due to the variation in values of resistance at the head, which consequently leads to producing an uneven image. Further, because only the image portions are locally heated at a high temperature, deformation of the substrate at the heated portions may be caused and, as a result, wrinkles tend to be generated. Also, as a means for improving uniformity in an image, a laser light can be employed for the heating in the heat transfer process. In this process, however, problems such as high cost of the apparatus, and a tendency to cause unevenness in the final image due to the decomposition of materials such as dyes or the like by a localized instantaneous heating at high temperatures have not been solved.
In contrast with the aforementioned image forming processes, an ink jet system, in which an image is formed by discharging ink as droplets, is used as a non-contact image forming process which does not affect the image receiving sheet or colorants used for the image forming. There are various types of ink jet systems such as a piezo-type, a thermal-type, a Hertz-type, and the like. These methods are disclosed in detail in the Journal of Science and Technology, Vol. 42, No. 1 (1996, USA). Here, a piezo-type will be described. A piezo-type ink jet systems includes a plurality of nozzle holes which consist of ink heads disposed in parallel, an independent discharge cavity which communicates with said nozzle holes and a portion of a wall of which works as a diaphragm, a piezoelectric element mounted on the diaphragm, and a common ink cavity which supplies ink into the discharge cavity. In the ink jet system, an image is formed on the image receiving sheet by applying pulse voltage in accordance with image information to the piezoelectric element, so as to discharge ink droplets from the nozzle holes.
In the ink jet system, problems such as the above-stated deformation of the substrate or decomposition of the coloring material due to the heat during recording can be avoided. The selection of the image forming materials, however, has been narrowly restricted due to liquid properties of the ink in order to prevent the clogging of the nozzle and stably form uniform ink droplets. When forming color images, dyes or specific pigments must be selected so as to prevent the nozzle clogging. As a result, the hue reproducibility in the formed image is restricted. For example, the ink jet system has not been able to be applied to printers for high accuracy printing proofs which are required to reproduce the same hues as those of printed ink pigments. Furthermore, even if the limited materials are employed, the nozzles may clog when the printer is not used for a long period of time. Also, the formed image has very poor light resistance and water resistance so that the dye tends to smear on the image receiving sheet.
An image forming process utilizing this ink jet system is disclosed in Japanese Patent Application Laid-Open (JP-A) No.11-70633 in which an image is formed by the following steps: applying, by an ink jet device, droplets of a cross linking agent image-wisely to a recording layer comprising a cross linking material; cross-linking and curing the portions to which the cross-linking agent has been applied; and removing, by washing, materials from the portions at which no cross-linking was performed. In this process, a plate for so-called screen printing is formed by using an ink jet, and then an image is formed by using colored ink. This process requires complex processing in that it is composed of two processing steps of plate-making and printing, and in that a development processing is needed in the plate-making step which causes problems such as the disposal of waste cleaning liquid and the like.
In addition, there is a so-called transfer-type ink jet system (e.g., Japanese Patent Application Laid-Open (JP-A) No. 5-42755) in which an image, which is formed from a recording solution including a liquid and a coloring agent, is formed on an image carrier by an ink jet recording system, and the image is transferred onto a body on which an image is to be transferred. This transfer-type ink jet system is an ordinary ink jet recording system in which, for example, an image including a dye is recorded once on an image carrier on a drum, and the image is then transferred onto a body on which an image is to be transferred. The same problems as with general ink jet systems remain, such as the limitations on the coloring materials, clogging of the nozzles and the like. Further, it is easy for smearing to occur, and only low resolution images can be obtained.
As an ink jet recording method using a transfer medium, JP-A-7-145576 discloses an image forming method in which an ink jet ink is, by an ink jet recording device, discharged image-wisely onto a transfer medium having a liquid-reactive resin layer as the uppermost layer thereof, the transfer medium is set in close contact with an image support, and the transfer medium and the image support are heated and pressed such that the image portions are transferred. This method is particularly suitable to cases in which the image support is formed from cloth. For example, when the ink is a water-based ink, the liquid reactive resin layer is formed from an aqueous resin, and when the ink droplets are discharged, the portions to which the ink droplets are applied dissolve and become tacky (adhesive). Due to this tackiness, only the portions to which the ink droplets are applied are transferred at the time of heating and application of pressure, and images having fine detail can be formed by using an ink jet method even on cloth. However, this method is especially directed to cases where the image support is cloth, and an ink jet ink is used. Thus, in the same way as regular ink jet recording, there are problems such as the coloring materials are restricted, the nozzles clog, and the like. Moreover, transfer is non-uniform, and there is unevenness in the transferred image.
In addition, Japanese Patent Application Laid-Open (JP-A) No. 62-117782 discloses an image forming method as follows. A solvent is applied imagewisely to the surface of an image carrier by using an ink jet discharging system. Thereafter, an ink layer is set in contact with the top surface of the image carrier, and only the portions of the ink layer contacting the image-wise solvent are formed on the image carrier. However, in this method, as is described on page 4, upper right column, lines 7 through 16 of JP-A-62-117782, a solvent having the appropriate cohesion and adhesion is required, and it is therefore difficult to select a solvent. Further, a synthetic resin film is used as the image carrier, but a synthetic resin film does not have good solvent acceptability as does a binder coated layer. Thus, it is extremely difficult to stably obtained good transferability. Moreover, in this method, reproduction of fine dots is insufficient, and multicolor transfer images in particular cannot be obtained at a high resolution.
JP-A-7-276780 is an improvement on the image forming method of above JP-A-62-117782. In JP-A-7-276780, as a method of applying a solvent image-wise onto a body to be recorded, a method is used in which heat energy is applied in accordance with image information to a porous body impregnated with solvent, and the impregnated liquid is related onto the body to be recorded as a mist (extremely fine droplets) or as vapor. Since this method does not use an ink jet discharging system, there is no clogging or the like. However, other than this application of mist or vapor to the body to be recorded, this method of JP-A-7-276780 is similar to the technique disclosed in JP-A-62-117782, and thus, the same problems as those described above arise.
In view of the aforementioned, it is an object of the present invention to provide an image forming process which is able to form an image having good hue reproducibility, and which overcomes problems such as restrictions on the image forming materials and clogging of the nozzles which are drawbacks of an ink jet system, and which suppresses damage to the substrate caused by heat and unevenness of the image which are drawbacks of a heat transfer system, and to provide an image forming process in which, in addition to the above points, transferability of fine points is good and a transferred image having a high resolution can be obtained, and to provide an image forming apparatus used for these image forming processes and can efficiently form a uniform image.
The present inventors have conducted intensive studies, and have found that an image which is uniform and has a good hue reproducibility can be formed by image-wisely applying a liquid substance which improves transferability to a transfer material which is used for a conventional heat transfer system, and then performing ordinary transfer processing. Thus, the present inventors arrived at the present invention.
The image forming process of the present invention is an image forming process including the steps of: (a) providing an image transfer material having a transfer recording layer formed from a heat transfer material and an image receiving sheet having a binder-resin-containing image receiving layer; (b) image-wisely applying a transferability promoting material to one of the heat transfer material and the image receiving sheet to form a latent image; and (c) applying pressure and heat to the image transfer material to form an image on the image receiving material from the latent image.
In the process described above, it is preferable that the temperature of heat processing in said transferring step is greater than or equal to the transfer temperature of the latent image portion and less than or equal to the transfer temperature of the heat transfer material. Further, it is preferable that the transferability promoting material which is used in the process is a composition which contains water and a nonionic surfactant. Further, it is preferable that the transferability promoting material used herein includes water and an organic solvent which is compatible with water and whose boiling point under normal temperature and normal pressure is 100xc2x0 C. or more. Further, in another aspect of the present invention, the image forming process of the present invention includes, in addition to the above steps, a step of transferring a transfer image, which has been formed on the image receiving sheet, onto another support.
In accordance with another aspect, the present invention is an image forming apparatus for use with a transferability promoting material and, an image transfer material having a transfer recording layer and a heat transfer material, and an image receiving sheet which has a binder-resin-containing image receiving layer and on which an image is formed, the apparatus comprising: (a) a droplet discharge head which image-wisely discharges droplets of transferability promoting material for lowering a transfer temperature of the heat transfer material onto one of the transfer recording layer and the image receiving sheet; and (b) a pair of pressure rollers for pressing the image transfer material and the image receiving sheet against one another, at least one of the rollers including a heater for applying heat. From the standpoint of operation, the image forming apparatus described above preferably further includes a peeling member for peeling the image forming material from the image receiving sheet after passing between the pressure rollers.